Solar calendar

A calendar that tracks the seasonal cycle of Earth’s orbit around the Sun, keeping its dates aligned with the solar year.

Tropical year: The interval between successive passages of the Sun through the vernal equinox, averaging about 365.2422 days. Its length sets the seasonal cycle that a solar calendar must track.
Earth's orbit: The path Earth follows around the Sun under their mutual gravitational attraction. Its annual cycle produces the changing solar position tracked by solar calendars.
Egyptian calendar: An ancient 365-day calendar organized into twelve 30-day months and five additional days. Its fixed year was an early solar scheme, though it slowly drifted against the seasons.
Lunar calendar: A calendar whose months follow the phases of the Moon, without necessarily tracking the solar year. Its months follow lunar phases rather than remaining fixed to the seasons.
Leap year: A calendar year with an extra day or other adjustment to keep dates aligned with an astronomical cycle. Adding a day periodically prevents seasonal dates from drifting through the calendar.
Seasons: Recurring annual periods shaped mainly by Earth’s axial tilt as it orbits the Sun. A solar calendar keeps its dates associated with these seasonal periods.
Julian calendar reform: Julius Caesar’s reform, introduced in 45 BCE, established a 365-day year with a leap day every fourth year. It replaced a drifting Roman calendar with a regular solar-year approximation.
Lunisolar calendar: A calendar that follows lunar months while adding adjustments to stay aligned with the solar year. It combines lunar month structure with seasonal alignment.
Leap day: An extra day inserted into a calendar year to keep it aligned with the solar year. In the Gregorian calendar, February 29 supplies the adjustment.
Solstice: Either of the two annual moments when the Sun reaches its greatest apparent distance north or south of the celestial equator. Solstices mark seasonal turning points used to define some solar years.

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